PTFE membranes, especially expanded PTFE membranes, are widely used for making breathable, waterproof fabrics. However, many PTFE membranes are dimensionally unstable due to the stress associated with manufacturing processes, including those described in U.S. Pat. No. 3,315,020 (hereafter “the '020 patent”). The '020 patent describes stretching a PTFE film unilaterally and bi-axially under tension, resulting in a membrane that has significant dimensional instability, especially at elevated temperatures such as experienced in fire fighting garments and many industrial filtration applications.
The dimensional instability of current stretched PTFE films can pose significant problems in applications where the PTFE membranes are used in composite constructions that contain layers of dissimilar materials. These dissimilar materials each exhibit dimensional changes at a much lesser degree to that of most conventional PTFE membranes under the same conditions. In such cases, products that incorporate a PTFE membrane as a component of a composite construction can develop inter-laminar shearing of the various layers under elevated ambient temperature conditions due to the different rates of expansion and contraction. Resulting effects of the above-described conditions are often a weak and/or unattractive product. In addition, product failure can occur prematurely by having delamination of the various layers.
Therefore, a need exists for a PTFE membrane having improved dimensional stability.